The paper presents the results of the tests on the preparation of iron (III) stearate - the most prominent oxo-biodegradation agent for polyolefinic plastics. The process chosen for testing was the double precipitation process, using stearic acid alkalised with soda lye and iron (III) chloride or iron (III) sulphate. A number of iron (III) stearate preparation methods described in the literature were tested and an analytical procedure for monitoring the process was developed using both the classical and instrumental techniques. FTIR spectroscopy is the most useful technique for the product quality control and identification. During the study the characteristics of the proper product were also determined using the DSC methods. Microscopy and laser diffraction particle size analysis were applied to determine the size of the precipitated product grains. An application sample of the product obtained by the double decomposition method from iron (III) sulphate as the iron carrier has successfully passed the application tests in the polyethylene packaging film.